1. Field of the Invention
The present invention relates to a discharge cathode device and a method for manufacturing the same for use with a plasma display panel or the like.
2. Description of Prior Art
A hexaboride such as a lanthanum hexaboride (LAB.sub.6) superior in electron radiation characteristic and ion impact resistance is known as a discharge cathode material for the discharge cathode device.
In Japanese Patent Laid-open Publication No. 55-62647, for example, there is described that a hexaboride (e.g., LAB.sub.6) is used as a cathode of a gas discharge display panel such as a plasma display panel, and an LaB.sub.6 film is formed on a base electrode such as nickel (Ni), wherein an operating voltage can be greatly reduced.
Among methods for forming the LaB.sub.6 film are a thick film printing method and a thin film method, etc. as described in the above Japanese Patent Laid-open Publication No. 55-62647. Also disclosed are an electron beam impact vapor deposition method and a sputtering method in Japanese Patent Laid-open Publication No. 61-253736 or Japanese Patent Laid-open Publication No. 3-101033.
In general, a sheet resistance in a cathode of a plasma display panel must be set to 0.1 .OMEGA. or less. Therefore, in the case where the cathode on a substrate solely comprises the LaB.sub.6 thin film, the thickness of the LaB.sub.6 thin film must be set to as great as tens of .mu. m, and the film tends to peel off the substrate, thus lacking practical applicability. To cope with this problem, as described in Japanese Patent Laid-open Publication No. 55-62647, a Ni plate as a base electrode for the LaB.sub.6 film is formed on the substrate to thereby reduce the thickness of the LaB.sub.6 film.
In the discharge cathode device for a plasma display panel or the like, the material of the base electrode for the LaB.sub.6 film must have the following properties.
(1) Good conductivity PA1 (2) Superior adhesion to the LaB.sub.6 film PA1 (3) Good flexibility, i.e., low rigidity (Since the LaB.sub.6 film receives great stress, it will break if it is not flexible.) PA1 (4) Resistance against oxidation in heat treatment (at 560.degree.-580.degree. C.) in the air to be performed later.
It is known that the base electrode of Ni can be formed by a thick film method or a thin film method. The Ni thin film is poor in flexibility and conductivity and not satisfactory in thermal oxidation resistance. Accordingly, if one wants the Ni film to be thickened so as to ensure a good conductivity, then the Ni film becomes more rigid and there arises a problem that the substrate will be broken in the heat treatment to be performed later.
Further, according to Japanese Patent Laid-open Publication No. 55-62647, a Ni thin film pattern and an LaB.sub.6 film pattern must be individually formed (i.e. collective patterning by means of etching is impossible), and it is difficult to accurately align the Ni thin film pattern with the LaB.sub.6 film one.
On the other hand, when a Ni thick film is used for the base electrode, it shows a good adhesion to the LaB.sub.6 film. However, since the surface of the Ni thick film is rough, the surface of the LaB.sub.6 film to be formed on the rough surface of the Ni thick film becomes also rough enough to cause poor adhesion therebetween. Consequently, that happens to cause the problem that the discharge initiating voltage is different from each other in each cell arranged in matrix array with both patterns of anodes and cathodes.